Microstructure evolution and the corresponding mechanical properties of Fe-Mn-Al-C-Nb low-density steel under aging treatment

• Synergistic Precipitation Behavior and Interfacial Relationship between NbC and κ-Carbides: This study systematically elucidated the synergistic precipitation characteristics and interfacial relationships of NbC and κ-carbides in Nb-microalloyed Fe-28Mn-10Al-C steel. With the increase in aging temperature from 450 °C to 550 °C, κ-carbides exhibited a morphological evolution from spherical to cubic forms and a change in their spatial distribution. Notably, κ-carbides tended to nucleate preferentially near NbC particles, despite the absence of a distinct crystallographic orientation relationship between the two phases. In contrast, κ-carbides displayed specific orientation relationships with the austenite matrix, namely: (011)κ//(022)γ, (100)κ//(200)γ, (011)κ//(011)γ. These observations provide new microstructural insights into the mechanisms by which microalloying elements regulate carbide precipitation behavior. • Microband-Induced Plasticity (MBIP) as the Dominant Deformation Mechanism under High Stacking Fault Energy (SFE): Comprehensive TEM and EBSD analyses at various stages of tensile deformation (up to fracture) demonstrate that dislocation slip is the dominant deformation mechanism in this high-SFE (84.31 mJ/m 2 ) steel. Specifically, plastic deformation is governed by the microband-induced plasticity (MBIP) mechanism. The microstructural evolution proceeds through the following sequence: dislocation accumulation → formation of Taylor lattice structures → development of high-density dislocation walls → microband formation. This mechanism accounts for the observed excellent strength–ductility balance (ultimate tensile strength of 1199 MPa and elongation of 37 %) and further refines the deformation theory for high-SFE Fe-Mn-Al-C steels.. • Quantitative Analysis of Strengthening Mechanisms with Dislocation Strengthening as the Primary Contributor: A quantitative theoretical analysis was performed to evaluate the contributions of various strengthening mechanisms, including lattice friction, solid solution strengthening, grain boundary strengthening, precipitation strengthening, and dislocation strengthening. The results indicate that dislocation strengthening is the principal contributor, accounting for approximately 32 %, 42 %, and 45 % of the total yield strength at 450 °C, 500 °C, and 550 °C, respectively. Its contribution increased substantially with aging temperature (from 307 MPa to 512 MPa), primarily due to increased dislocation density caused by interactions with evolving precipitates. In contrast, precipitation strengthening (σ p ) exerted a relatively minor effect, contributing only 33–34 MPa. This quantitative evaluation provides critical guidance for the alloy design and process optimization of high-strength, low-density steels for automotive and structural applications. To develop high-strength, low-density steels applicable for automotive field, the study systematically investigated Fe-28Mn-10Al-C-0.5Nb steel after aging at 450 °C-550 °C in terms of its microstructure evolution, mechanical properties, and deformation and strengthening mechanisms. Electron backscatter diffraction (EBSD) method served for examining the austenite grain morphology and the orientation of annealing twins at different aging temperatures. Transmission electron microscopy (TEM) served for elucidating the precipitation behavior and spatial distribution of NbC, κ-carbides, and other secondary phases. Furthermore, the deformation mechanisms under different tensile strains were explored using TEM and EBSD, with particular attention to the evolution of dislocations and other substructures in the deformed specimens. Quantitative evaluation was conducted on the yield strength variation under varying strengthening mechanisms through theoretical modeling. According to relevant results, with rising aging temperature, the finely dispersed spherical κ-carbides gradually transform into a uniformly distributed rectangular morphology. The strength and toughness of the experimental steel both increase with aging temperature, and the steel aged at 500 °C exhibits an outstanding overall property, with a tensile strength of 1199 MPa and an elongation of 37 %. Planar dislocation slip is the primary deformation mode, and the favorable strength-ductility balance results from the microband-induced plasticity. Calculations confirm dislocation strengthening as the primary strengthening mechanism in the experimental steel.